The present invention relates to improvements in the method of semiconductor production and in the apparatus for implementing that method. More specifically, the invention relates to a method of position detection and to a method and apparatus of printing miniature patterns by use of that position detection method, the latter method and apparatus being used illustratively in X-ray exposure systems, steppers and electron beam lithography systems to improve their alignment accuracy.
To print miniature patterns such as a semiconductor integrated circuit with lithography techniques (e.g., reduction projection, X-ray exposure, electron beam lithography) requires accurately detecting the position of a sample placed on the sample stage and then aligning the sample precisely with a reticle or the like. Along with a growing demand in recent years for higher density in semiconductor devices has come the increasing need for higher overlay accuracy. Today, the overlay accuracy is required to be as high as 0.05 .mu.m or less in error.
The alignment between sample and reticle generally involves the use of position detection marks on the top surface St (this and other reference characters are identified in the accompanying drawings) of the sample (i.e., the surface on which patterns are printed). In that case, there exist limits to improving the overlay accuracy, because there may occur an alignment error attributable to resist coating irregularities or a damaged position detection mark. This disadvantage appears to have been circumvented by a method, proposed in Japanese Patent Laid-Open No. 55-46053, whereby position detection marks are provided on the bottom surface Sb of the sample (i.e., on the surface on which patterns are not printed). Some other methods, such as one in Japanese Patent Laid-Open No. 63-160722 and another in Japanese Patent Laid-Open No. 63-224327, also utilize the marks on the sample bottom surface Sb for alignment. These methods involve detecting the positions of position detection marks on the sample bottom surface Sb and, in alignment with those mark positions, printing the corresponding patterns on the top surface St of the sample.
The method of detecting the mark positions on the sample bottom surface Sb poses new problems that need to be solved, illustratively when an overlay accuracy level of 0.03 .mu.m required of devices since the 0.2 .mu.m rule is to be achieved. That is, those error factors that used to be ignored without adverse effects become no longer negligible. When the sample tilts and its surface causes misalignment with a reference surface, the thickness of the sample generates a difference between the mark-bearing bottom surface Sb and the top surface St on which to print patterns. That difference leads to a detection error that cannot be ignored when the overlay accuracy is 0.03 .mu.m.
One conventional way to correct the detection error is to install a tilt detection optical system that detects the tilt angle of the sample. One disadvantage of this solution is that adding the tilt detection optics makes the manufacturing equipment larger and more complicated. Thus the development of a position detection method has been desired which would detect marks formed on the sample bottom surface, with no use of the tilt detection optics, in order to readily correct the position error attributable to a tilt of the sample. Because of its location, the detector for detecting the bottom surface marks would need to be embedded in the stage that bears the sample. This in turn would require the detector to be sufficiently small in size. These requirements have not been met successfully with the prior art.
In a setup where mark positions are detected from the bottom surface of the sample, the stage that supports and moves the sample is obviously located below the sample. In order to secure the sample onto the stage, at least part of the bottom surface of the sample must remain in snug contact with the top surface of the stage. In this setup, it is impossible to detect any mark position from that part of the sample bottom surface which is in contact with the stage top surface; the mark positions must be detected from the sample bottom surface except for the contact part thereof. Thus there are two major constraints on mark detection from the sample bottom surface. One constraint is that it is necessary to locate a mark position detector within the limited space opposite to the sample bottom surface that is not in contact with the stage top surface. The other constraint is that it is impossible to detect the positions of the sample bottom marks in the area contacting the stage. For example, where the sample is vacuum chucked onto the stage, the marks on the sample bottom surface being chucked cannot be detected. Although there has been proposed a method that uses an optical mark position detector in conjunction with a stage transparent to the mark detecting light, this method also has a disadvantage. That is, optical detection means cannot detect those sample bottom marks which correspond to the vacuum chucking grooves on the stage.
The need to keep as flat as possible the sample top surface on which to print patterns makes it desirable to distribute vacuum chucking positions as evenly and as widely as possible on the sample bottom surface. Thus the stage top is provided with regularly spaced windows (on the surface not contacting the sample bottom surface) or transparent portions (i.e., transparent to the light for optical mark detection). Only those marks which correspond to the windows or to the transparent portions are optically detected. Where position detection marks are to be conventionally detected from the sample top surface, these marks are formed at the same time as the patterns of a semiconductor circuit or the like. This means that detecting the positions of major marks is equivalent to detecting the positions of major patterns. Where the position of a sample bottom mark is to be conventionally detected, it is customary to begin the detecting process by finding the position of the sample top pattern corresponding to the bottom mark position. At this point, the mark position is readily detected if one condition is met, i.e., if the spacing of the windows or transparent portions on the stage coincides with, or is an integer multiple of, the chip size or pattern pitch of a semiconductor integrated circuit or the like to be formed on the sample top surface. If this condition is not met, mark position detection is not available. This difficulty is conventionally overcome only through the tedious replacement of the current stage with another that has vacuum chucking windows (or grooves) appropriately spaced to match the chip size for optical position detection.
In sum, little consideration has been apparently given to the above-described drawbacks in the previously proposed methods involving putting position detection marks on the sample bottom surface and detecting those marks to determine the position of the sample top surface. That is, those methods have done little to eliminate the need for switching sample stages when detecting the precise position of the sample top surface for alignment with new patterns to be formed on that surface in conjunction with the sample's chip size or pattern pitch for the semiconductor integrated circuit or the like in question.